The present invention relates to brazing materials and methods for repairing components that operate at high temperatures. More particularly, this invention relates to repairs made with a braze material containing a nickel-base alloy whose composition is suited for the repair of nickel-base and cobalt-base superalloys of the type used to form combustor liners of gas turbine engines.
High temperature cobalt-base and nickel-base superalloys are used in the manufacture of components that must operate at high temperatures, such as combustor and turbine components of gas turbine engines. During engine operation, these components are subject to strenuous high temperature conditions under which various types of damage or deterioration can occur. For example, combustor liners suffer cracks that typically initiate at surface irregularities such as igniter tubes or large diameter dilution holes, and propagate as a result of stresses that are aggravated by thermal cycling. Because the cost of components formed from high temperature cobalt and nickel-base superalloys is relatively high, it is typically more desirable to repair these components than to replace them.
Repair methods for combustor liners forged from superalloys have included tungsten inert gas (TIG) welding techniques. In conventional air-cooled combustor liners, relatively large dilution holes (e.g., diameters of 0.20 inch (about 5 mm) or more) spaced apart more than ten times the hole diameter are typical. TIG welding has been well suited for such repairs as a result of necessitating only limited post-weld work, while being practical in view of the relatively large spacing and size of the dilution holes. However, with improvements in the efficiencies of gas turbine engines, improved cooling methods have become necessary. Such methods typically involve reduced cooling air flow applied more uniformly over the cooled surfaces. This, coupled with increased combustion inlet temperatures and the development of materials with greater fatigue strength, has led to the development of combustor liners that employ transpiration film cooling, in which a far greater number of much smaller cooling holes are uniformly dispersed at the liner surface. Transpiration cooling holes are precision formed by such methods as laser machining to be inclined to the liner surface and closely spaced to produce uniform film cooling over the flow path surfaces of the liners, thus reducing thermal damage from hot combustion gases. However, the use of transpiration film cooling has complicated the field repair of cracks in combustor liners from service distress. Though often still initiating at surface irregularities, cracking typically progresses through multiple transpiration holes. Because of the smaller hole size and spacing, conventional weld repair can be expensive as a result of the filling and destruction of a large number of transpiration holes in and adjacent the repair site, requiring restoration of the holes by laser drilling or electrical-discharge machining (EDM).
A more recent and cost-effective approach developed for the repair of superalloy components is termed activated diffusion healing, or ADH, which involves a vacuum brazing operation. The ADH process employs an alloy powder or mixtures of powders that will melt at a lower temperature than the superalloy component to be repaired. If two powders are combined, one of the powders is formulated to melt at a much lower temperature than the other powder, such that upon melting a two-phase mixture is formed. The vacuum brazing cycle causes the braze powder mixture to melt and alloy together and with the superalloy of the component being repaired. A post-braze diffusion heat treatment cycle is then performed to promote further interdiffusion, which raises the remelt temperature of the braze mixture.
With the advent of higher strength and more highly alloyed superalloys, improved repair materials have been required that are specialized for the particular superalloy to be repaired. It is often the intent to provide a braze alloy that will result in a repair characterized by high strength and a microstructure that is closely matched with the microstructure of the article being repaired. As a result, a considerable variety of braze alloy materials have been developed for use in the ADH process and other braze repair techniques. While many highly suitable repair materials have been formulated to perform well with various high strength cobalt-base and nickel-base superalloys, the prior art lacks a braze repair material that is especially formulated to repair combustor liners formed from certain superalloys. Of primary concern here, braze repair materials for liners must be uniquely tailored to the mechanical and environmental properties required for the particular liner to be repaired, whose property requirements will depend on the type of engine and its application, whether aerospace or industrial.
The present invention provides a braze material and method for repairing an article, such as gas turbine engine combustor liners formed from nickel-base and cobalt-base superalloys. The braze material is composed of a nickel-base braze alloy that is preferably in powder form and dispersed in a suitable vehicle, such as a binder that can be chosen to form a slurry, putty or a solid tape with the powder. The binder serves to adhere the braze alloy particles together, as well as adhere the particles to the article to be repaired. Alternatively, the braze alloy particles can be sintered together to yield a rigid repair preform.
According to this invention, the braze alloy is formulated to be capable of withstanding the high temperature operating environment of a combustor liner, and to have a melting temperature below the grain growth or incipient melting temperature of the superalloy to be repaired. A preferred braze alloy is formed by combining at least two nickel-base powders. A first of the powders consists essentially of, in weight percent, 10 to 18chromium, 6 to 14 cobalt, 3.5 to 6.5 titanium, 1.5 to 4.5 aluminum, 2.5 to 5.5 tungsten, 2.5 to 5.5 molybdenum, 0.05 to 0.30 carbon, 0.01 to 0.20 zirconium, 0.001 to 0.2 boron, wherein the combination of tungsten and molybdenum is at least 5.0, the balance being nickel and incidental impurities. The second powder consists essentially of, in weight percent, 10.0 to 20.0 chromium, 2.0 to 5.0 boron, up to 3.0 iron, the balance being nickel and incidental impurities. A suitable weight ratio of the first powder to the second is about 50:50 to 75:25. A suitable combined composition for the braze alloy is, in weight percent, about 10 to about 19 chromium, about 3 to about 10.5 cobalt, about 1.75 to about 4.9 titanium, about 0.75 to about 3.4 aluminum, about 1.25 to about 4.1 tungsten, about 1.25 to about 4.1 molybdenum, about 0.025 to about 0.225 carbon, about 0.005 to about 0.15 zirconium, about 0.50 to about 2.6 boron, up to 1.5 iron, with the balance being nickel and incidental impurities. As used herein, incidental impurities are those elements that may be difficult to completely eliminate from the braze alloy due to processing limitations, yet are not present in sufficient quantities to significantly alter or degrade the desired properties of the alloy.
According to this invention, a method for repairing a superalloy combustor liner entails filling cracks, voids or other distressed surface regions of the liner with the braze material, after which the liner is heated in a vacuum environment to a temperature sufficient to cause the braze alloy to melt, flow and fill the cracks/voids, and to promote wetting and alloying of the braze alloy with the superalloy of the liner, such that a metallurgical bond results upon cooling. For the braze alloy of this invention, a temperature of not more than about 2200xc2x0 F. (about 1200xc2x0 C.) is sufficient to melt and flow the braze alloy without causing grain growth or incipient melting of the liner superalloy. Thereafter, the superalloy preferably undergoes a heat treatment to promote further interdiffusion between the braze alloy and the liner superalloy.
According to this invention, the braze alloy is ideally formulated to be compatible with certain superalloys that have been specially developed for combustor liners for gas turbine engines, and particularly those used in aerospace applications. As a repair material, the braze alloy exhibits excellent wettability of cracks and voids with nickel-base and cobalt-base superalloys, and the resulting repairs exhibit stress-rupture properties that meet or exceed that of liners repaired by conventional TIG methods. A preferred composition for the braze material has the additional advantage of having a sufficiently high viscosity at braze temperatures to allow the braze alloy to flow into and fill cracks and other distressed areas, yet inhibits the molten alloy from flowing and plugging surrounding cooling holes. This advantage has been observed even where cracks intersect small and closely-spaced transpiration cooling holes of an air-cooled liner.
Other objects and advantages of this invention will be better appreciated from the following detailed description.